If you hook the output up to the guino injector lead, you should not get anywhere near 20ma out of it.
I would just try putting 12 volts accross it (if it is rated for at least 12v) and connecting the output to the injector-in connecter on the guino, then wrapping the injector lead around it or around a ferrous core of some sort to amplify the signal.
Then try and scope what voltages are reaching the CPU pins.
If it works, yippie
, If you get an inverted gph, then that is promising, can fix in software.
Unfortunately I have neither pulse and hold or diesels at my disposal so I have to grope around a bit in these areas.